Constant look inside to make certain space fenced off (9)
I believe the answer is:
enclosure
'space fenced off' is the definition.
'constant look inside to make certain' is the wordplay.
'constant' becomes 'c' (mathematical abbreviation).
'look' becomes 'lo'.
'inside' means one lot of letters goes inside another.
'to make certain' becomes 'ensure' ('ensure' can be a synonym of 'make certain'**).
'c'+'lo'='clo'
'clo' inserted within 'ensure' is 'ENCLOSURE'.
(Other definitions for enclosure that I've seen before include "Document included with a letter" , "compound" , "Area fenced off - one's ulcer (anag)" , "Pen" , "pound" .)